In general, a railroad vehicle is characterized in that because it travels by means of its iron wheels rolling over rail surfaces, its traveling resistance is smaller than that of an automobile. In particular, in a recent electric railroad vehicle, regenerative brake control is performed in which the main motor is operated as an electric power generator at the braking mode so that braking force is obtained and in which electric energy generated by the main motor at the braking mode is returned to the overhead line so as to be recycled as power running energy for other vehicles. It is known that an electric railroad vehicle equipped with a regenerative brake travels with approximately half energy consumption in comparison with an electric railroad vehicle that is not equipped with any regenerative brake; thus, the foregoing method utilizes the characteristics of a railroad vehicle that has a small traveling resistance.
Meanwhile, it is the present situation that in a local railroad or the like where the transportation density is low, meticulous passenger service is realized at low cost, by utilizing a diesel railcar that does not require any infrastructure such as an overhead line or a substation. However, because a diesel railcar has no means for transferring energy to other vehicles through an overhead line or the like, the recycle of regenerative energy, which is performed by an electric railroad vehicle, has not been carried out. Accordingly, it has been considered that in order to realize energy saving in a diesel railcar, there is no choice but to rely on the development of a high-mileage engine.
As a method of promoting energy saving even for such a diesel railcar, there has been proposed a hybrid diesel railcar in which an engine, an electric power generator, and an electric storage device are combined with one another (e.g., refer to Patent Document 1). In the hybrid diesel railcar, because an electric storage device is provided, it is made possible that at the braking mode, the electric storage device temporarily absorbs generative energy generated by the main motor, and the absorbed regenerative energy is given to the main motor so as to be recycled as part of energy required at the power running mode, so that the energy saving can be realized.
The conventional hybrid diesel railcar disclosed in Patent Document 1 is configured with an engine, an AC power generator that is driven by the engine so as to output AC power, a converter that converts AC power into DC power, an inverter that inverts DC power into AC power, a motor that drives a railroad vehicle, a speed reducer that reduces the rotation of the motor and transfers it to a wheel axle, an electric storage device that has a function of performing charging and discharging with DC power, a service-power-source inverter, a service-power-source transformer, and a control apparatus.
In the hybrid diesel railcar disclosed in Patent Document 1, as far as the output characteristics for the rotation speed of the engine is concerned, when the rotation speed of the engine becomes higher than a given rotation speed, the output of the engine is reduced, and when the rotation speed of the engine becomes lower than a given rotation speed, the output of the engine is raised; the speed command for the converter is utilized in implementing constant-power generation control with a constant load amount, regardless of the rotation speed.